Method and apparatus for RFID communication

ABSTRACT

A plurality of battery-operated transceivers encapsulated by lamination to form a sheet of independent transceivers is tested in a two piece fixture that forms an enclosure surrounding each in-sheet transceiver. Each enclosure has an antenna for transmitting a command signal to the transceiver at a known power level and for receiving a reply message from the transceiver containing a power level measurement made by the transceiver. Test methods using the fixture of the present invention are also described.An RFID tag and interrogator may each include a transmitter and a receiver. The tag and interrogator may communicate with each other at different frequency bands and may communicate in accordance with a wireless communication protocol.

RELATED REISSUE APPLICATIONS

More than one reissue application has been filed for the reissue of U.S.Pat. No. 6,487,681. The reissue applications are the initial reissueapplication Ser. No. 10/997,556 filed Nov. 24, 2004, a continuationreissue application Ser. No. 11/864,708 filed Sep. 28, 2007, acontinuation reissue application Ser. No. 11/864,710 filed Sep. 28,2007, a continuation reissue application Ser. No. 11/864,715 filed Sep.28, 2007, another continuation reissue application Ser. No. 11/864,723filed Sep. 28, 2007 and the present continuation reissue application.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of a reissue application Ser. No.10/997,556, filed Nov. 24, 2004, which is a reissue application of U.S.patent application Ser. No. 09/437,718, now U.S. Pat. No. 6,487,681,which is a continuation of application Ser. No. 08/306,906 filed Sep.15, 1994, now U.S. Pat. No. 5,983,363, which is a continuation in partof and claims priority from U.S. patent application Ser. No. 07/979,607filed Nov. 20, 1992, now U.S. Pat. No. 6,058,497, and the teachings ofall of which are incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to transponder testing and to test systems,fixtures, and methods for testing transponders.

BACKGROUND OF THE INVENTION

As an introduction to the problems solved by the present invention,consider the conventional transponder used for radio frequencyidentification (RFID). Such a transponder includes a radio transceiverwith a built-in antenna for receiving command message signals and fortransmitting reply message signals. Inexpensive transponders findapplication in systems for tracking material, personnel, and animals,inventory management, baggage handling, and the mail to name a few majorareas.

A transponder necessarily includes a transceiver. Such transponders mayinclude an integrated circuit transceiver, a battery, and a printedcircuit antenna hermetically encapsulated in a laminated package about 1inch square and approximately as thick as a mailing label or tag. Insuch a laminated package, manufacturing acceptance tests on each unitbecome difficult and costly.

Conventional transponders are inexpensively manufactured in sheetshaving for example 250 integrated circuit transceivers spaced apart in arow and column array between polymer films. Prior to use, thetransponders are separated from each other by shearing the sheet betweenadjacent rows and columns. Conventional testing methods and apparatuscannot be used until the transponders are separated from each other.

Conventional manufacturing acceptance tests for transponders are basedin part on antenna performance tests that simulate the application inwhich the transponder will be used. These so called “far-field” testsrequire a large anechoic chamber and individual testing of a singletransponder at a time. Such far-field testing adds significantly to theper unit cost of inexpensive transponders.

Without inexpensive transponder testing for manufacturing acceptancetests, incomplete testing may perpetrate unreliable tracking, inventory,and handling systems, increase the cost of maintaining such systems, anddiscourage further development and popular acceptance of transpondertechnology.

In view of the problems described above and related problems thatconsequently become apparent to those skilled in the applicable arts,the need remains in transponder testing for more accurate and lesscostly test systems, fixtures, and test methods.

SUMMARY OF THE INVENTION

Accordingly, a test system in one embodiment of the present inventionincludes a fixture, an interrogator, and a switch cooperating fortesting a sheet containing a plurality of transceivers, each transceiverwithin a contour on the sheet. The fixture, in one embodiment, admits asheet of transceivers and surrounds each transceiver at its contour sothat each transceiver is respectively enclosed within an enclosure.Within each enclosure is an antenna for so called “near-field”communication. The interrogator determines a command signal andevaluates reply signals from each transceiver. The switch is coupled inseries between each antenna and the interrogator for selecting anantenna for transmitting the command signal and for receiving the replysignal.

According to a first aspect of such an embodiment, the fixture isolatestransceivers from each other so that multiple transceivers are testedsimultaneously. By isolating each transceiver, interference fromadjacent transceivers is minimized, transponder identity and locationare not confused, and test transmissions are prevented from affectingexternal equipment including other test stations.

According to another aspect, testing is facilitated by isolating eachtransceiver at its contour.

According to another aspect, multiple transceivers are moved as a sheetand tested without further handling so that rapid testing is feasibleand delays for physical alignment of the transceivers within the fixtureis minimized.

According to another aspect, near-field testing is used to eliminate theneed for large chambers.

According to another aspect of such a test system, the transfer functionof the antenna and detector portion of a transceiver receiver is tested.

The present invention is practiced according to a method in oneembodiment which includes the steps of providing an enclosure thatadmits a sheet of transceivers, each transceiver formed within arespective region of the sheet, closing the enclosure to form an RF sealabout each respective region, and operating each transceiver forreceiving and transmitting signals.

According to a first aspect of such a method, independent testing ofindividual transceivers is accomplished for in-sheet transceivers andmultiple transceivers are tested simultaneously.

According to another aspect, far-field tests are used to confirm thetest signal used in manufacturing tests.

A method, in an alternate embodiment, for testing battery-operatedtransceivers includes the step of transmitting a wake up signal to atransceiver. According to a first aspect of such a method, onlytransceivers under test are made operational so that battery power isconserved in other transceivers.

These and other embodiments, aspects, advantages, and features of thepresent invention will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the art byreference to the following description of the invention and referenceddrawings or by practice of the invention. The aspects, advantages, andfeatures of the invention are realized and attained by means of theinstrumentalities, procedures, and combinations particularly pointed outin the appended claims.

An RFID tag and interrogator may each include a transmitter and areceiver. The tag and interrogator may communicate with each other atdifferent frequency bands and may communicate in accordance with awireless communication protocol.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a test system of the present invention.

FIG. 2 is a functional block diagram of the test system of FIG. 1.

FIG. 3 is a functional block diagram of a transponder of the presentinvention to be tested in the test system of FIG. 1.

FIG. 4 is a cross sectional view of fixture 15.

A person having ordinary skill in the art will recognize where portionsof a diagram have been expanded to improve the clarity of thepresentation.

In each functional block diagram, a broad arrow symbolically representsa group of signals that together signify a binary code. For example, agroup of bus lines is represented by a broad arrow because a binaryvalue conveyed by the bus is signified by the signals on the several buslines taken together at an instant in time. A group of signals having nobinary coded relationship is shown as a single line with an arrow. Asingle line between functional blocks represents one or more signals.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a plan view of a test system of the present invention. Testsystem 10 provides manufacturing acceptance tests for an in-sheettransponder 12 provided on continuous roll 20 of laminated films.Transponders under test are located in fixture 15. Tested transpondersare received on roll 22. Fixture 15 is connected by cable 18 tosubsystem 24 so that signals generated by instrumentation in subsystem24 are coupled to fixture 15 and so that signals received in fixture 15are coupled to instruments in subsystem 24 for analysis. Subsystem 24includes interrogator 25 and computer 86, cooperating for signalgeneration and analysis. Fixture 15 is characterized, according to amethod of the present invention, using a correlation to far-fieldtesting. Characterization of a system, fixture, or circuitconventionally includes making measurements of characteristic featuresof its structure and operation.

Transponders to be tested in an alternate embodiment are provided tofixture 15 in separated sheets, each sheet having an array of rows andcolumns of transponders. For example in one embodiment, about 250transponders are manufactured in a sheet measuring about 18 inches byabout 24 inches.

Test system 10 also includes materials handling equipment, not shown,for supplying sheets or rolls of transponders for testing, for aligningtransponders within fixture 15, and for receiving tested transpondersfor further manufacturing steps. In one embodiment, individual testedtransponders are separated (singulated) from the sheet in which testingoccurred and are provided on an adhesive backing for distribution astape-and-reel components or ready-to-use articles such as baggage tags,inventory labels, or badges, to name a few feasible applications.

Roll 20 includes a plurality of identical transponders, such astransponder 12. Transponder 12 is a radio frequency identification(RFID) device of the type described in U.S. patent application Ser. No.07/990,918 by Snodgrass et al. filed Dec. 15, 1992, incorporated hereinby reference. In one embodiment, transponder 12 is about 1 inch square,includes a lithium battery, an integrated circuit transceiver, and anantenna packaged using thin film and lamination techniques.

FIG. 2 is a functional block diagram of a test system of the presentinvention. Test system 10 includes six major functional elements:operator console 26, test system computer 86, interrogator 25, radiofrequency (RF) switch 92, fixture 15, and material handling apparatus90.

In operation, test system computer 86 directs material handlingapparatus 90 to align a sheet of transponders (not shown) within fixture15. Alignment assures that each transponder is isolated from othertransponders in a manner to be discussed with reference to FIG. 4. Inone embodiment, alignment includes automatic recognition by video cameraof guide marks on the sheet and control of stepper motors according tosoftware performed by computer 86 or in an alternate embodiment by acomputer in material handling apparatus 90. One of ordinary skill willrecognize that alignment includes the location of the fixture relativeto the sheet so that the fixture, the sheet, or both can be repositionedto accomplish proper alignment.

When a sheet of transponders is aligned, computer 86 directs RF switch92 for independently testing individual transponders. In a firstembodiment, one transponder is tested at a time. In an alternateembodiment, multiple interrogators are coordinated to test multipletransponders simultaneously. Independent transponder operation duringsimultaneous testing of multiple transponders is accomplished in part byisolation provided by fixture 15.

During tests of each transponder, computer 86 directs interrogator 25,particularly interrogator central processing unit (CPU) 84, to generateand transmit via transmitter 82 command messages through switches 91 and92, and to receive and interpret reply messages generated by thattransponder that are conveyed through RF switch 92 and switch 91 toreceiver 83. Interrogator 25 is of the type described in U.S. patentapplication Ser. No. 07/990,918 by Snodgrass et al. filed Dec. 15, 1992,incorporated herein by reference. Switch 91 and switch 92 are coaxswitches, common in the RF testing art. In alternate embodiments, switch91 is eliminated and command and reply messages are separated bycommunication techniques known in the art, for example separation bytime division or use of different frequency bands or differentmodulation techniques.

In one embodiment of the present invention, a test of the sensitivity ofthe receiver portion of the transceiver portion of a transponder undertest includes transmitting from interrogator 25 a test signal, forexample, a command message at a test power level. Transponders that failto respond by transmitting a proper reply message fail the test at afirst point. In another embodiment, the reply message includes ameasurement of the signal strength seen by the receiver portion of thetransponder under test. Transponders that report measurements ofreceived signal strength that do not exceed an expected signal strengthfail the test at a second point. By setting both test points asrequirements, the population of tested transponders is of higher qualitybecause marginal units are rejected. Therefore, the determination of thetest power level and the expected signal strength are important toproduction and application economics.

Fixture 15 surrounds each transponder so that each transceiver's antennais within one enclosure. In one embodiment, the enclosure surrounds anentire transponder and a small volume of ambient air so that theenclosure forms a cavity. In an alternate embodiment, only thetransceiver's antenna is enclosed. In yet another alternate embodiment,the small volume is filled with potting material so that, for example,the cleanliness of the enclosure and the position of the antenna withinthe enclosure are maintained. In one embodiment, the potting materialincludes polyimide. In alternate embodiments, conventional pottingmaterials and conventional materials used for films for encapsulatingthe transponder are used. The power level to be used for each soenclosure depends on the materials and dimensions of the enclosure andthe transponder.

To determine the test power level appropriate for one of severalenclosures formed by fixture 15, far-field test results are correlatedto conventional characterization tests of the transponder, pottingmaterial (if any), and the enclosure. By repeating characterizationtests in each enclosure, a so called cavity transfer function relatingtest power level to received signal strength is determined for eachenclosure of fixture 15.

FIG. 3 is a functional block diagram of a transponder of the presentinvention to be tested in the test system of FIG. 1. Transponder 12includes battery 120, antenna 110, transceiver 115, multiplexer 122,analog to digital (A/D) converter 124, and central processing unit (CPU)126. Transceiver 115 includes transmit/receive switch 112, receiver 114,and transmitter 128. Transponder 12 operates from battery power providedby battery 120. All functional blocks are coupled to receive batterypower signal VB.

In operation, CPU 126 directs multiplexer 122 to select one of severalanalog signals for conversion. For example, when a report of batteryvoltage is desired, line 121 is selected and coupled to A/D converter124. In response to a signal on line 123, A/D converter 124 provides adigital signal on line 125 to CPU 126. CPU 126 then forms a messagesignal on line 127 and directs transmission by transmitter 128 throughswitch 112 and antenna 110.

Except for antenna 110 and battery 120, the circuitry of transponder 12is conventionally formed as an integrated circuit, manufactured in largenumber on a wafer. In a preferred test method of the present invention,some manufacturing acceptance tests are conducted after fabrication of awafer containing perhaps a thousand independent integrated circuits. Forexample, the conversion accuracy of A/D converter 124 varies from waferto wafer depending on variations in the fabrication process. Prior toforming dice from the wafer, all or a representative sample of A/Dconverters, are tested by introducing stimulus signals and obtainingresponse signals via wafer probes, as is well known in the art. Testresults are generalized to determine an A/D transfer function relatingsignals 123 and 125 for the A/D converters on a particular wafer.

Operation of transponder 12 includes at least two modes of operation. Ina first mode, power is conserved by disabling most transponder circuits.When a wake up signal is received by antenna 110, coupled to receiver114 through switch 112, detected and demodulated by receiver circuit118, and interpreted by CPU 126 as a proper wake up signal, transponder12 enters a second mode of operation. In the second mode, power isapplied to substantially all transponder circuitry for normal operation.In a preferred embodiment, the test signal is both a wake up signal anda request for a report of received signal strength.

Receiver 114 includes detector 116 for detecting received signalstrength. Antenna 110 is coupled through switch 112 to convey an RFsignal on line 130 to detector 116. Detector 116 provides on line 117 tomultiplexer 122 signal RSS1 proportional to received signal strength.When a report of received signal strength is desired, line 117 isselected and signal RSS1 is coupled to A/D converter 124. In response toa signal on line 123, A/D converter 124 provides a digital signal online 125 to CPU 126. CPU 126 then forms a message signal on line 127 anddirects transmission by transmitter 128 through switch 112 and antenna110.

FIG. 4 is a cross sectional view of fixture 15. Fixture 15 includesfirst section 14, second section 16, and an antenna in each enclosure(or cavity). For example, cavities 71, 72 and 74 are shown with antenna66 in cavity 72. First section 14 includes a matrix of ridges, forexample 52 and 56. Second section 16 includes a matching matrix ofridges, for example 54 and 58. Each pair of ridges for example 56 and 58separates and defines adjacent cavities, for example cavities 72 and 74.

The upper surface of ridges 54 and 58 in second section 16 define ahorizontal plane onto which a portion of roll 20 of laminated films ispositioned. When that portion includes in-sheet transponders, materialhandling apparatus position the portion for in-sheet transpondertesting. First section 14 and second section 16 are then pressedtogether against sheet 20 so that each transponder, for exampletransponder 51, is isolated from each other transponder in sheet 20.Ridges about each cavity form an RF seal.

The RF seal provides isolation. Isolation prevents RF energy radiatedfrom antenna 66 in cavity 72 from interfering with tests conducted inadjacent cavity 74. The RF seal is not perfect and, therefore, isolationis not perfect, due to leakage for example between ridges 52 and 54 andbetween 56 and 58. Since leakage RF energy must pass through films 44and 46, conventional shielding in the neighborhood of the contactbetween adjacent ridges is effective to further reduce leakage andthereby improve isolation. Such shielding includes placement ofconductors and conductive materials within, between, and on the surfacesof films 44 and 46.

Isolation is operative to decouple an antenna in one enclosure from anantenna in an adjacent enclosure. From the point of view at antenna 66,when a signal originating in cavity 72 is stronger than a signaloriginating in cavity 74, for example, the signal sources and theirrespective antennas are considered decoupled from each other. Decouplingcan also be accomplished by improving the gain of cavity 72, forexample, by making its dimensions compatible with a wavelength of thesignal originating in cavity 72.

In an alternate embodiment, first section 14 and second section 16 arefabricated as flat plates having no ridges 52, 54, 56, or 58. Thedistance between these plates is smaller than one wavelength of thesignal originating in cavity 72 so that adjacent transponder antennasare effectively decoupled for purposes including manufacturingacceptance testing. In such an embodiment, first section 14 and secondsection 16 sandwich the sheet therebetween.

In a preferred embodiment, each transponder is formed within a squarecontour and each cavity has a matching square cross section so thattransponders are isolated each one at its contour. In this sense, acontour extends through both films 44 and 46 to circumscribe onetransponder. In a mathematical sense, a contour is defined on a surface.Since top film 44 has an upper surface, a first contour is defined onthat top surface. Since bottom film 46 has a bottom surface, a secondcontour is defined on that bottom surface. The square cavity formed byridges 54 and 58 in the second section is circumscribed by a thirdcontour in the plane defined by the tops of the ridges on which thesheet is positioned. Thus, alignment includes positioning the sheet andthe fixture so that the third contour formed on ridges 54 and 58 touchesthe sheet at the second contour on the bottom of film 46. When properlyaligned, the first section, having a similar fourth contour on ridges 52and 56, touches the first contour on the top of film 44. In a preferredembodiment, the first and second contours are directly opposed throughthe sheet. In alternate embodiments, ridges 52 and 54 touch film 44along a sloped, concave, notched, or stepped surface for greaterisolation. In such embodiments, important contours are not necessarilydirectly opposed.

Transponder 51 is identical to transponder 12 as previously described.Transponder 51 is of the type described as an enclosed transceiver inU.S. patent application Ser. No. 08/123,030, filed Sep. 14, 1993,incorporated herein by reference. The cross-sectional view oftransponder 51 shows integrated circuit 48 and battery 50 between film44 and film 46. Integrated circuit 48 includes the transceiver circuitryof transponder 51. Battery 50, in one embodiment, includes a metalsurface coupled to operate as part of the antenna for the transceivercircuitry. Additional conductive traces on film 44 and film 46 cooperatefor coupling battery power to integrated circuit 48 and for operation aspart of the antenna for the transceiver. Films 44 and 46 are sealed toeach other around a contour that encircles integrated circuit 48 andbattery 50. In one embodiment, the seal is made by embossing so that thethickness of films 44 and 46 is reduced as shown at seal 42. Aftertesting, transceiver 51 is separated from the sheet by cutting throughfilms 44 and 46 at a point outside seal 42 so that transceiver 51remains sealed after testing.

The central internal conductor of coax cable 70 is extended into cavity72 for operation as a near-field antenna. Feed through fitting 68 holdscoax cable 70 onto second section 16, shields the central conductor, andprovides continuity of impedance from cable 70 up to antenna 66.

The amount of radiation coupled between antenna 66 and transponder 51depends in part on several variables including the dimensions of cavity72, the wavelengths of the radiated signals, potting or other materials(if any) within the enclosure, and the distance between antenna 66 andfilm 46. Although the location of transponder 51 is controlled bymaintaining tension on sheet 20 as first section 14 is pressed againstsecond section 16, these variables are expected to vary to some extentfrom cavity to cavity, from test to test, and over time with wear andhandling of fixture 15 and operation and wear in materials handlingapparatus used to position fixture 15, sheet 20, or both.

In a preferred embodiment, antenna 110 of transponder 12 is a squareloop antenna for communication at about 2.45 gigahertz. The wavelengthat that frequency is about 12.2 centimeters or about 4.82 inches. One ofordinary skill in the art will understand that cavity dimensionsdiscussed above must lie outside the loop antenna. Conventionalsimulation may be used to arrive at sufficient or optimal dimensions ofthe cavity and sufficient or optimal dimensional characteristics of theantenna, including its placement and type (dipole, loop, stub, Marconi,etc).

According to a method of the present invention, the magnitude of signal117 as shown in FIG. 3 is determined so that the effect of variation inthe variables discussed above is removed from transponder test resultsand the pass rate for tested transponders is improved. Such a methodbegins with a first step of characterizing the encapsulated transponderwith far-field tests. Before transponder 51 is tested in fixture 15, thedigitization transfer function for analog to digital converter 124 shownin FIG. 3 is determined in a second step. As with the first step, inthis second step 1, a desired level of accuracy for manufacturingacceptance tests is achieved using one of several approaches includingdesign simulation, theoretical analysis, tests of a prototype, tests ofrepresentative samples, or tests of every transponder. In a preferredembodiment, sufficient accuracy is obtained for a manufacturing lot oftransponders by conducting wafer probe tests for the second step.

In a third step, the cavity is characterized by design simulation,theoretical analysis, or conventional tests.

Fourth, a prototype or representative transponder 51 is placed in thecavity shown in FIG. 4 that was characterized in the third step. In afifth step, a pass/fail test power level and the expected reportedsignal strength are determined by analysis of the results of tests madewith the representative transponder, the characterization data, and theresults of simulation and other techniques known in the art. Togetherthe process of determining in this fifth step is defined as correlatingfar-field measurements with transceiver responses.

After test power level and response data are determined, manufacturingacceptance testing can proceed by replacing the representativetransponder with an untested transponder 51. While in the cavity andisolated from other transponders, several tests are performed includinga receiver sensitivity test.

A receiver sensitivity test of the present invention includes thefollowing steps: radiating a test signal from antenna 66; convertinganalog signal RSS1 received by antenna 110 to a digital result on line125; transmitting, by means of transmitter 128 and antenna 110, amessage conveying the digital result; receiving the message via antenna66; and making a pass/fail determination based on the response (if any)from the untested transponder. As one result, defects in antenna 110,switch 112, and receiver circuit 118 are made apparent.

The foregoing description discusses preferred embodiments of the presentinvention, which may be changed or modified without departing from thescope of the present invention.

For example, the orientation and shape of fixture 15 as two plates asshown in FIGS. 1 and 4 in alternate and equivalent embodiments aremodified for cooperation with material handling apparatus, not shown. Inone such modified orientation, the plane at which first section 14 andsecond section 16 meet is vertical rather than horizontal. In one suchmodified shape, the fixture has a spherical shape (rather than generallyhexahedral), each contour surrounding a transponder is circular (ratherthan square), and each cavity is spherical (rather than generallyhexahedral). In other embodiments, antenna 66 is located in variouspositions including, for example, in an opposite section of a cavity,within a ridge, in an adjoining cavity not completely isolated byridges, or (for multiple antennas per cavity) at several of theselocations.

Still further, those skilled in the art will understand that firstsection 14, second section 16, or both in alternate and equivalentembodiments are formed along an axis of turning to permit advancing aportion of sheet 20 as a portion of the fixture turns about its axis. Inone embodiment, such movement moves and aligns sheet 20.

In a preferred embodiment, a microwave frequency band is used fortransponder communication. The same band is used for transpondertesting. In alternate embodiments that a person skilled in the art withknowledge of the teachings of the present invention would recognize asequivalents, another one or more frequency bands are utilized.

As still another example, the complexity of transponder 12 shown in FIG.3 in alternate embodiments is simplified. Without departing from thescope of the present invention, for example, transmitter 128 is replacedwith a transmitter responsive to an analog instead of a digital input,receiver circuit 118 is replaced with a circuit providing an analograther than a digital output, analog to digital converter 124 iseliminated and CPU 126 is replaced with an analog rather than a digitalcircuit.

These and other changes and modifications known to those of ordinaryskill are intended to be included within the scope of the presentinvention.

While for the sake of clarity and ease of description, several specificembodiments of the invention have been described; the scope of theinvention is intended to be measured by the claims as set forth below.The description is not intended to be exhaustive or to limit theinvention to the form disclosed. Other embodiments of the invention willbe apparent in light of the disclosure to one of ordinary skill in theart to which the invention applies.

The words and phrases used in the claims are intended to be broadlyconstrued. A “system” refers generally to electrical apparatus andincludes but is not limited to rack and panel instrumentation, apackaged integrated circuit, an unpackaged integrated circuit, acombination of packaged or unpackaged integrated circuits or both, amicroprocessor, a microcontroller, a memory, a register, a flip-flop, acharge-coupled device, combinations thereof, and equivalents.

A “signal” refers to mechanical and/or electromagnetic energy conveyinginformation. When elements are coupled, a signal is conveyed in anymanner feasible with regard to the nature of the coupling. For example,if several electrical conductors couple two elements, then the relevantsignal comprises the energy on one, some, or all conductors at a giventime or time period. When a physical property of a signal has aquantitative measure and the property is used by design to control orcommunicate information, then the signal is said to be characterized byhaving a “magnitude” or “value.” The measure may be instantaneous or anaverage.

The following disclosure corresponds to the Detailed Description Sectionand figures of U.S. Pat. No. 5,365,551 (the '551 patent) incorporated byreference above. FIG. 1 of the '551 patent is a functional block diagramof communication system 30 of the present invention as described in the'551 patent. In FIG. 1 of the '551 patent, commander stations 10 and 34and responder stations 40 and 36 are coupled to common medium 32 bynetwork interfaces 26 and 60, respectively. In practice, a plurality ofcommander and responder stations would be distributed geographically.The type of medium selected for communication depends on thecommunication system application; see below for equivalent variations.The embodiment depicted in FIG. 1 of the '551 patent illustrates theinvention in an application such as airport baggage handling. For thisembodiment, the medium is free space through which radio frequencycommunication are transmissible.

Commander station 10 is designed to achieve a flexible systemarchitecture while incorporating many commercially available components.Commander station 10 includes personal computer system 12 having dataand control bus 14 shared by central processor 16, memory 18, andperipheral controllers 20a and 20c. Monitor 22, disk drive 24, andnetwork interface 26 connect to individual peripheral controllers20a-20c via connecting signals 28a-28c, respectively. Network interface26 is coupled to common medium 32. Network interface 26 could beimplemented in a chassis separate from the chassis of personal computersystem 12 or equivalently could be implemented in combination with thefunctions of the network interface peripheral controller 20c forconnection directly to data and control bus 14.

The configuration of commander station 10 illustrates severaladvantages. Communication system configuration and operation are largelydictated by software loaded via disk drive 24, stored in memory 18, andperformed by central processor 16. Disk drive 24, memory 18, centralprocessor 16, as well as monitor 22 and peripheral controllers 20a20care all conventional, general purpose, and readily available apparatus.Therefore, additional functions and changes to communication system 30can be made in software with little or no mechanical changes tocommander station 10. The operation of commander station software willbe discussed below.

Responder station 40 is designed for minimal circuitry to achieve, amongother things, small size and low power consumption. Small size permitsconvenient use, for example, as a baggage tag. Low power consumptionpermits further size reduction and reduces manufacturing and operatingcosts. Small size and low manufacturing costs combine to permitimplementing responder station 40 as a convenient, dispensable,throwaway item such as a baggage tag, package label, or the like.

In essence, microsequencer 42 forms the core of responder station 40.Microsequencer 42 is a read only memory that produces data signal 48 inresponse to address signal 44. In operation, a value is presented asaddress signal 44 once every period of clock signal 46. Data signal 48from microsequencer 42 is stored in state register 50 once every periodof clock signal 46. The output of state register 50 is state signal 52,which forms control bus 54. Control bus 54 causes register transferoperations to be described below. A portion of state signal 52 defines aportion of address signal 44. Thus, a sequence of state transitionsoccurs in synchronism with clock signal 46 as defined by the internaloperation of microsequencer 42 and other signals together comprisingaddress signal 44, as will be described below. A state transitiondiagram is also discussed below. In the typical microsequencer, internalmultiplexers reduce the range of read only memory addresses that wouldotherwise be required. Microsequencer 42 is of a class of devicesdescribed by Charles Belove in “Handbook of Modern Electronics andElectrical Engineering,” pp 2135-2142, published by John Wiley & Sons,New York, N.Y. (1986), incorporated herein by reference; and by Y. Chuin “Computer Organization and Microprogramming,” published byPrentice-Hall, Englewood Cliffs, N.J. (1972) incorporated in full hereinby reference.

Network interface 60 of responder station 40 is coupled to common medium32 in a way similar to the coupling of network interface 26 of commanderstation 10 to common medium 32. Network interface 60 connects to stateregister 50 to supply clock signal 46. Network interface 60 connects tocontrol bus 54 so that send and receive operations are directed in partby state signal 52. When microsequencer 42 is in an appropriate state,data received by network interface 60 is transferred from networkinterface 60 to command register 56 by data bus 62 in conjunction withload signal 58. Command register output 59 defines a portion of addresssignal 44. Network interface 60 also connects via data bus 62 to memory64, register array 66, flag register 84, and random number generator 90for transfer of data between these function blocks, for storage ofreceived data, and for recall of data to be sent.

In one embodiment, data bus 62 is byte-wide. Network interface 60converts received data from serial to byte-parallel organization. Theseveral devices that connect to data bus 62 make a byte-parallelconnection.

In another embodiment, data bus 62 is bit-serial. Control bus 54, insuch an embodiment, includes serial clock signal (not shown), Registertransfer among network interface 60, register array 66, memory 64, flagregister 84, and random number generator 90 are accomplished inbit-serial fashion with appropriate electrical interfaces known in theart.

In yet another embodiment, a combination of serial and parallel datapaths are implemented. The system designer's choices of serial orparallel as well as the number of bits per register transfer operation,depends on factors including system and device timing limitations, noiseimmunity, power dissipation, device size, topology, and layoutconstraints.

Memory 64 connects to read/write signal 68 and memory address signal 70which are part of control bus 54. Memory 64 is used to store values forresponder station identification and data related to the communicationsystem application. For example, when a responder station is used as anairline baggage tag, postal mailing label, or inventory control tag,memory 64 would store data describing a destination for the item towhich the tag is attached.

Register array 66 performs functions similar to a multi-port memory.Register array 66 connects to arithmetic-logic unit (ALU) 72 for thepresentation of operand signals 74 and 76, and storage of result signal78. Operand and result signals are multi-bit digital signals forarithmetic operations such as addition, bit-wise parallel logicaloperations such as logical-AND, and bit-wise serial operations such asshift-left. Register array 66 connects to control bus 54 so thatregisters to be coupled to operand and result signals are selected andstored according to state signal 52.

In addition to the connections already described, a portion of controlbus 54 connects to ALU 72 to supply opcode signal 80 to ALU 72. Opcodesignal 80 selects one of a plurality of possible operations to beconducted by ALU 72. When an equality comparison has been selected byopcode signal 80 and operand signals 74 and 76 are bitwise identical,A=B signal 82 is asserted by ALU 72. A=B signal 82 defines a portion ofaddress signal 44.

Control bus 54 connects to individual bits arranged in flag register 84.Addressed-bit 86, part of flag register 84, is set under control ofstate signal 52 to indicate whether responder station 40 has beenaddressed in a received command message. Locked-bit 88, also part offlag register 84, is set under control of state signal 52 to indicatewhether responder station 40 should ignore messages from a commanderstation because responder station 40 has already announced itsidentification to a commander station. The significance of addressed-bit86 and locked-bit 88 will become more readily apparent in thedescription below.

Random number generator 90 connects to control bus 54 and data bus 62for transferring a random number of a predetermined precision toregister array 66. When retained in register array 66, the random numberis called an ARBITRATION NUMBER whose function will be discussed below.Circuit techniques for generating a random number in digital format arewell known and described, for example, by H. F. Murray in “GeneralApproach for Generating Natural Random Variables”, IEEE Transactions onComputers, Vol. C-19, No. 12, pp 1210-1213, December 1970, incorporatedherein by reference. In one embodiment, random number generator 90 issimilar to an integrated circuit implementation described by AlanFolmsbee, et. al., in their article, “128K EPROM with Encrypted ReadAccess”, published in the Digest of Technical Papers IEEE InternationalSolid-State Circuits Conference pp 46-47 and 103, by Lewis Winner, CoralGables, Fla., 1985, incorporated herein by reference.

FIG. 2 of the '551 patent is a functional block diagram of networkinterface 26 shown in FIG. 1 of the '551 patent. Within networkinterface 26, connecting signal 28c couple to output buffer 110.Byte-parallel loading of output buffer 110 is accomplished by networkinterface peripheral controller 20c shown in FIG. 1 of the '551 patent.Bytes are removed from output buffer 110 by transmitter logic 112 toaccomplish several processing objectives. In one embodiment, a 5-bitcyclic redundancy check (CRC) code is joined to each 8-bit byte to forma 13-bit word. Redundancy, provided by the 5-bit code, facilitates errordetection and limited error correction by responder station 40. Table 1describes the format of the 13-bit word and includes a description ofthe eRC code used in one embodiment. Suitable CRC encoder and decodercircuits used in transmitter logic 112 and receiver logic 178 aredescribed in detail in “Error Control Coding: Fundamentals andApplications,” by Shu Lin and Daniel J. Costello Jr., Prentice-HallEnglewood Cliffs, N.J. 1983, pp 62-94. Transmitter logic 112 alsogenerates transmit serial bit stream 114 which includes a messagepreamble bit stream, one or more successive 13 bit words from outputbuffer 110, and a postamble bit stream. When permitted by OK-to-transmitsignal 116, transmit serial bit stream 114 is presented to transmitter118. Transmitter 118 in one embodiment produces a radio frequencytransmit signal 120 by modulation and couples that signal to antenna122. Appropriate modulation methods depend on the communication medium.

TABLE 1 Bit Order of Transmission D7, D6 . . . D0, P4, P3, . . . PO (POtransmitted first) CRC Generation Equations P0 = 01 + 02 + 05 + 07 P1 =01 + 03 + 04 + 06 P2 = 0 + D2 + 03 + 06 + 07 P3 = 00 + 04 + 05 + 06 + 07P4 = 00 + 01 + 02 + 03 + 04 + 05

For a communication system for airport baggage handling, modulationincludes, for example, spread spectrum modulation having pseudo noisecharacteristics. Other techniques for transmitter design appropriate toradio transmission and other media will be readily apparent to thoseskilled in the arts applicable to communication on a particular medium.A power level of approximately 1 watt is sufficient to excite responderstation network interface 60 at distances and noise levels typicallyrequired for a communication system for airport baggage handling.

Receiver 124 is coupled to antenna 122 for amplifying and filteringradio frequency received signal 126. Receiver 124 derives OK-to-transmitsignal 116 from power level measurements on received signal 126 andprovides signal 116 to transmitter logic 112. Although responder stationnetwork interface 60 need not generate a transmitted signal using thesame modulation technique employed in transmitter logic 112 andtransmitter 118, a common method is preferred, for example, in order tominimize circuitry in responder station network interface 60. Thus,receiver 124 removes the carrier signal and other artifacts ofmodulation generated by responder station network interface 60 in oneembodiment by synchronizing with the spread spectrum signal and removingpseudo noise characteristics through known detection and filteringmethods. Resulting received serial bit stream 128 is coupled to receiverlogic 130, in one embodiment, for determining whether a proper messagehas been received and for decomposing the message into successive 8 bitbytes. The method and circuitry required to determine whether a propermessage has been received depend on the redundancy that responderstation network interface 60 incorporates into received serial bitstream 128.

For a communication system for airport baggage handling, responderstation 40 may transmit at a power level of 1 milliwatt or less.Multiple and more sophisticated error detection schemes transmitted fromresponder station 40 will extend the limit of physical separationbetween commander station 10 and responder station 40. Error detectionschemes are well known. Such schemes may also permit reliablecommunication in environments having substantial noise levels. On theother hand, limits to the complexity and power consumption of responderstation 40 may limit the extent of encoding circuitry therein. Whereresponder station network interface 60 facilitates one or moreparticular error detection schemes, receiver logic 128 decomposes,decodes, detects, and to a limited extent corrects errors in receivedserial bit stream 128. In one embodiment, receiver logic 130 determinesproper-message-received signal 132 by decoding Viterbi encoding usingmodel Q1601 decoder available from Qualcomm, Inc., San Diego, Calif.according to Qualcomm application notes and the parameters: Rate R=½,Generating Functions GO=171(octal) and G1=133(octal), and ConstraintLength K=7. Receiver logic 130 also performs serial to parallelconversion to produce successive 8-bit bytes which are stored in inputbuffer 134.

Network interface peripheral controller 20c, responsive toOK-to-transmit signal 116 and proper-message-received signal 132,generates signals on data and control bus 14 from which centralprocessor 16 can determine whether a proper message has been received asof a predetermined time after transmission and if not, whether nomessage was received. Other control signals, known in the art and notshown, are generated and sensed to orchestrate the loading of outputbuffer 110 and the unloading of input buffer 134 under control ofcentral processor 16.

FIG. 3 of the '551 patent is a functional block diagram of responderstation network interface 60 shown in FIG. 1 of the '551 patent. Theconfiguration illustrated in FIG. 3 of the '551 patent performsfunctions similar to those already described above for commander stationnetwork interface 26. Differences between the two serve primarily tolimit the complexity of responder station circuitry. Data bus 62connects to transmitter logic 160. When directed by microseguencer 42via signals on control bus 54, transmitter logic 160 generates a messagepreamble bit stream. Then, for each bit of each word read from memory 64or register array 66, transmitter logic 160 develops a Viterbi code.Functional descriptions suitable for designing circuits or computerprograms for generating Viterbi and similar convolutional codes areexplained in “Error Control Coding: Fundamentals and Applications,” byShu Lin and Daniel J. Costello Jr., PrenticeHall Englewood Cliffs, N.J.1983, pp 287-456; and “Error-Correction Coding for DigitalCommunications”, by George C. Clark, Jr. and J. Bibb Cain, Plenum Press,New York, N.Y. 1981, pp 227-266. Message signal 162 presents the codesto transmitter 164. Following the last code, transmitter logic 160generates a message postamble bit stream. Transmitter 164 modulatesmessage signal 162 in a way compatible with receiver 124 and receiverlogic 130. The resulting transmit radio frequency signal 166 is coupledto antenna 168. Redundancy, provided by the Viterbi codes, facilitateserror detection and limited error correction when the message isreceived at commander station 10.

Receiver 170 is coupled to antenna 168 for amplifying and filteringradio frequency received signal 172. Receiver 170 derives wake-up signal174 from power-level measurements on received signal 172 and provideswake-up signal 174 to power control and restart circuits not shown. In acommunication system for airport baggage handling, non-criticalcircuitry in responder station 60 is powered by battery only after thepreamble of a packet has been detected. Receiver 170 also removes thecarrier signal and other artifacts of modulation generated by commanderstation network interface 26 in one embodiment by synchronizing with thespread spectrum signal and removing pseudo noise characteristics throughdetection, demodulation, decoding, and filtering methods known in theradio communication arts. Resulting received serial bit stream 176 iscoupled to receiver logic 178. In one embodiment, receiver logic 178performs several functions: Determining whether a proper byte has beenreceived, consequently generating improper-byte-received signal 180, anddecomposing the packet into successive 8-bit bytes forming receivedmessage signal 182. The method and circuitry required to determinewhether a proper byte has been received depend on the redundancy thatcommander station network interface 26 incorporates into transmit serialbit stream 114. Receiver logic 178 detects the first byte of a commandand in response generates load signal 58. Clock signal 46 is alsogenerated by receiver logic 178 to drive state register 50.

Microsequencer 42 and network interface 60 cooperate via control bus 54which includes improper-byte-received signal 180. Other signals includedin control bus 54, known in the art and not shown, orchestrate transferof bytes between memory 64, register array 66, transmitter logic 160,and receiver logic 178. If an improper byte is received, as indicated byimproper-byte-received signal 180, microsequencer 42 responds byreverting to an idle state and ignoring incoming bytes until anothercommand is received.

For a detailed description of suitable circuits of the type that can beused for transmitters 118 and 164 and receivers 124 and 170, implementedin spread spectrum technologies, see U.S. Pat. No. 5,121,407 by Partykaet al., incorporated herein by reference.

FIG. 4 of the '551 patent is a diagram of the packet format sent bycommander station 10 to responder station 40. Each command packet 140includes, in order of transmission, a preamble followed by a commandfollowed by a postamble. The preamble and postamble are designed forsynchronizing a transmitter circuit and a receiver circuit for aparticular packet.

In one embodiment, the preamble bit stream comprises 768 ‘1’ bitsfollowed by a 7-bit Barker code of ‘0001101’. In one embodiment, thepostamble comprises a 7-bit Barker code of ‘111001 0’.

In one embodiment, each bit of the command format is modulated using apseudo noise (PN) sequence for direct sequence spread spectrumcommunication. The sequence is generated in part by a linear feedbackshift register (LFSR) of the form [5,2]. In this form, the input to thefirst of five registers is the result of combining the output ofregister 5 by exclusive-OR with the output of register 2. The generatorin this embodiment has 32 states so that the 1 and 0 states occur withequal probability. Since the LFSR generates only 31 states, anadditional state is inserted by support circuitry. For a detaileddescription of a suitable PN modulator circuit of the type employed intransmitter 118 see “Spread Spectrum Systems”, by R. C. Dixon, publishedby John Wiley and Sons, Inc. 1984 pp 1528 and 56-151 incorporated hereinby reference. Suitable demodulator techniques and circuits (of the typeused in receiver 170 to recover the response format) are also describedat pages 153-290 incorporated herein by reference.

FIG. 5 of the '551 patent is a table that describes several commands andrefers to command formats described in FIG. 6 of the '551 patent. Asshown in FIG. 6 of the '551 patent, each command begins with an opcodeand has one of four formats varying in length from 3 bytes to 258 bytes.Opcode values were selected to facilitate accurate decoding and obtainhigh noise immunity. Each byte is an 8-bit word as it would appear inoutput buffer 110 and on received message signal 182. The opcodehexadecimal value is stored on receipt in command register 56. Bytesfollowing the opcode have the following meanings. MASK and BRANCH asused in format 142 are binary numbers chosen by a commander station tospecify a group of responder stations that should act on the command andshould reply. LOCAL ID in format 142 is a unique identification numberassigned, for example, by the communication system installer to eachcommander station 10, 34 coupled to common medium 32. Responder stations36, 40 coupled to common medium 32 can then direct a response to one ofseveral commander stations 10,34 by, for example, including a particularLOCAL ID in each response. When one commander station chooses to specifyonly one responder station that should act on a command and shouldreply, that commander station includes in its command an ARBITRATIONNUMBER as in formats 144 and 146 identifying the responder station. AnARBITRATION NUMBER is a short value, for example 1 byte, chosen for selfidentification by a responder station. On the other hand, A TAG, as informat 146, is a long value, for example 8 bytes, assigned by acommunication system designer at the time a responder station ismanufactured or commissioned. The ARBITRATION NUMBER distinguishesresponder stations when coupled simultaneously with at least onecommander station to a common medium. However, the TAG, distinguishesresponder stations throughout the life of the communication systemapplication. Finally, DATA in format 146 includes some or all of thecontents for any or all devices including memory 64, register array 66,flag register 84, and random number generator 90.

FIG. 7 of the '551 patent is a diagram of the packet format sent byresponder station 60 to commander station 10. Each response packet 190includes, in order of transmission, a preamble followed by a responsefollowed by a postamble. The preamble and postamble are designed forsynchronizing a transmitter circuit and a receiver circuit for aparticular packet. In one embodiment, the preamble bit stream comprises768 ‘1’ bits followed by a 7-bit Barker code of ‘0001101’. In oneembodiment, the postamble comprises a 7-bit Barker code of ‘1110010’.

In one embodiment, each bit of the response format is modulated using apseudo noise (PN) sequence for direct sequence spread spectrumcommunications. The sequence is generated in part by a linear feedbackshift register (LFSR) of the form [6,1] or [8,4,3,2] for either a 64chip sequence or a 256 chip sequence respectively. In the form [6,1],the input to the first of six registers is the result of combining theoutput of register 6 by exclusive-OR with the output of register 1.Similarly, for the [8,4,3,2] form, the input to the first of eightregisters is the result of the exclusive-OR of the outputs of registers8, 4, 3, and 2. The 64 chip sequence requires less time for signalsynchronization than the 256 chip sequence; however, the latter providesbetter performance in systems having poor signal to noise ratio. Thegenerator in this embodiment has an even binary multiple of states, sothat the 1 and 0 states occur with equal probability. Since the LFSRgenerates one less state, an additional state is inserted by supportcircuitry. For a detailed description of a suitable PN modulator circuitof the type employed in transmitter 164, see “Spread Spectrum Systems”,by R. C. Dixon, published by John Wiley and Sons. Inc. 1984 pp 1528 and56-151 incorporated herein by reference. Suitable demodulator techniquesand circuits of the type used in receiver 124 to recover the responseformat are also described at pages 153-290 incorporated herein byreference.

FIG. 8 of the '551 patent is a table that describes several responsesand refers to response formats described in FIG. 9 of the '551 patent.As shown in FIG. 9 of the '551 patent, response formats 192-196 includeLOCAL ID, ARBITRATION NUMBER, and TAG, which have the meanings alreadydescribed above. By including LOCAL ID and ARBITRATION NUMBER in eachresponse, in cooperation with locked bit 88 one responder station canrespond unambiguously to one commander station in the presence of aplurality of commander and responder stations. The INVERTED ARBITRATIONNUMBER in format 192 is the binary ones-complement of the ARBITRATIONNUMBER and is included for increased accuracy of communication. REVISIONin format 192 is a one-byte value set by a communication systemdeveloper at the time of manufacture or commissioning of a responderstation. REVISION represents the responder station configuration andconnotes its capability. STATUS in format 196 is a one-byte code chosenby responder station 40 to convey current conditions of important systemevents such as low battery, uncorrectable data received, writeprotection. And similar information which may indicate to commanderstation 10 that communication should be repeated or abandoned. DATA inresponse format 194 includes some or all of the contents of any or alldevices including memory 64, register array 66, flag register 84, orrandom number generator 90.

A communication system, according to the present invention as describedin the '551 patent, includes commander and responder stations thatadhere to a method of communicating called a protocol. In general, theprotocol of the present invention as described in the '551 patent placesdifferent requirements on a commander station than on a responderstation. Thus, there is a commander station method (FIG. 10 of the '551patent) and a responder station method (FIG. 11 of the '551 patent).These methods together implement the communication system protocol.

Operation according to the present invention as described in the '551patent produces the following characteristic effects at the systemlevel. First, a commander station will not begin transmitting during thetransmission by another commander station or by a responder station.Operation, according to the present invention as described in the '551patent, does not prevent more than one commander station from beginningtransmission simultaneously; however, it is feasible to couple commanderstations to a second medium or to constrain commander stations to asecond or expanded protocol on common medium 32. For example, commanderstations 10 and 34 include personal computer system 12, which can beaugmented with a peripheral controller for operation over ethernet.Communication over the second medium can be used to prevent simultaneousbroadcast over common medium 32. For example, a second protocol oncommon medium 32 may include operator action to assign time slots, backoff delays, or similar means for media access whether central ordistributed. Several embodiments for these means for media access havebeen described by Stallings in his work already incorporated byreference above.

Second, a responder station will not transmit unless it has firstreceived a command to which it determines it must respond. The responseis made within a predetermined time immediately following receipt of thecommand.

Third, a commander station can form a command in a manner calling forall, more than one, or one responder station to respond. An importantobject of the communication system protocol in a communication system ofthe present invention as described in the '551 patent, i.e.uninterrupted communication, is achieved after a commander stationdetermines how to cause only one responder station to respond. Theprogram flow diagram of FIG. 10 of the '551 patent and the state diagramof FIG. 11 of the '551 patent describe how uninterrupted communicationbetween one commander station and each responder station is achievedwhen a plurality of commander stations and a plurality of responderstations are simultaneously coupled to a common medium.

FIG. 10 of the '551 patent is a program flow diagram of the protocolfollowed by a commander station of the present invention as described inthe '551 patent. A practical example of a communication system will beused to describe the flow diagram.

In a communication system for airport baggage handling the quantity andidentity of responder stations within the radio communication range of acommander station varies over time. A commander station may be at afixed operator station within radio range of a moving belt conveyingbaggage toward a V-junction of conveyors. When baggage tags areconstructed as responder stations and when each tag has destinationinformation stored in memory 64, the commander station, throughcommunication with each baggage tag, can control the routing of each bagthrough the junction onto one of two conveyors. Assume that eachresponder station also has information in memory 64 describing itssequential position on the conveyor. Such a sequence number could be adate and time of day when the bag passed through a chute upstream of thecommander station.

As a group of bags approaches the commander station, the commanderstation has a fixed amount of time to determine the identity of eachresponder station, in order to establish uninterrupted communication.For proper baggage handling, the commander station must routinely andrepeatedly identify all bags on the conveyor. To do so, at FIG. 10 ofthe '551 patent block 210, commander station 10 specifies a group ofresponder station addresses by choosing values for BRANCH and MASK.BRANCH and MASK values are determined in a manner to be explained byreference to FIG. 12 of the '551 patent below. In one embodiment, theinitial group specification, i.e. BRANCH and MASK values, would specifyall possible responder stations. Commander station 10 at block 212generates an “identify, clear, and generate” (IDCG) command having aformat according to FIGS. 4,5, and 6 of the '551 patent. When the mediais clear to broadcast, block 214, as indicated by OK-to-transmit signal116, the IDCG message is broadcast, block 216. An IDCG message causeseach responder station that is a member of the group to clear locked-bit88, generate a random number and retain it as its ARBITRATION NUMBER,and broadcast a response. The responder station's reactions to 10, lOG,IDC, and IDCG commands are explained further in reference to FIG. 11 ofthe '551 patent below.

Commander station 10 now loops through blocks 220 and 218 for a responseto be received as indicated by OK-to-transmit signal 116 or a time outelapsed condition. If a response was received, as indicated by a falsestate of OK-to-transmit signal 116, commander station 10 at block 222determines whether a collision occurred, as indicated by a false stateof proper-message-received signal 132. If commander station 10determines that a collision occurred, it will determine at block 224whether all possible members of the initial group of responder stationaddresses specified at block 210 have been addressed in an 1D, lDG, IDC,or IDCG command. How this determination is made will be furtherexplained with reference to FIG. 12 of the '551 patent below. If allsubgroups have not been tried, the commander station again specifies agroup of responder station addresses, for example, a subgroup ordisjoint group of a prior group. At block 228 commander station 10generates an 10 command according to FIGS. 4, 5, and 6 of the '551patent and continues the method from block 214.

If, at block 218, a predetermined time elapsed without a false conditionappearing on OK-to-transmit signal 116, commander station concludes thatno response was transmitted and continues the method at block 224.

If, at block 222, the proper-message-received signal is true, thencommander station 10 concludes that only one responder stationresponded. At block 230, commander station 10 determines and validatesthe responding responder station's ARBITRATION NUMBER according toresponse format 192 using ARBITRATION NUMBER and INVERTED ARBITRATIONNUMBER. According to a particular system communication objective,commander station 10 now selects a command from FIG. 5 of the '551patent which will cause the responder station to set its locked-bit 88.For determining baggage destination and positional sequence on theconveyor, commander station 10 could select RD. Using the appropriatecommand format shown in FIG. 6 of the '551 patent, commander station 10generates a message at block 232, loops until the OK-to-transmit signalindicates that the medium is clear to broadcast at block 234, thenbroadcasts the command at block 236. Commander station 10 again awaits aproper response message by looping at block 238 through block 240. If apredetermined time elapses at block 240, commander station 10 continuesthe method at block 234. If a response is received without error atblock 244, as indicated by proper-message-received signal 132, then twoparty uninterrupted communication between commander station 10 and oneresponder station 60 has been established. Further communication may berequired, as indicated by the STATUS code in the received responseformat 192 or to accomplish other system communication objectives.

It is possible at block 224 for the commander station to determine thatno further subgroup, super group, or disjoint group remains to becommanded using the 10 command. Suppose, for example, that all practicalvalues of BRANCH and MASK have been used. If the immediately precedingbroadcast at block 216 elicited no response at block 218, then commanderstation 10 can conclude that all responder stations have beenidentified. Otherwise, at block 248, commander station 10 generates anidentify and generate command (lOG) according to the format in FIGS. 4,5, and 6 of the '551 patent using the same group that was specified inblock 210. Commander station 10 continues the method at block 214.

Although the same group is specified, a responder station that has beenidentified at block 244 will not respond, since its locked-bit 88 hasbeen set. Collisions are less likely to occur with each pass through theloop from block 214 to block 248 because a smaller number of responderstations can respond. Hence, the method of FIG. 10 of the '551 patentconverges toward identifying all responder stations. The communicationsystem designer must select the precision of BRANCH and MASK values toassure conversion within system time allowances, for example, 8-bitBRANCH and MASK values are compatible with conveyor speeds and radioranges needed for airport baggage handling.

FIG. 11 of the '551 patent is a state diagram of the protocol followedby a responder station of the present invention as described in the '551patent. Responder station 40, begins in idle state 310 when power isapplied or restored according to wake-up signal 174. In part, the idlestate is indicated by contents of command register 56 not correspondingto a valid command. The idle state is re-entered to interrupt commandprocessing when improper-byte-received signal 180 is raised by receiverlogic 178. A valid command loaded into command register 56 causes statetransition to address check state 312.

In address check state 312, microsequencer 42 determines whetherresponder station 40 has been addressed by one of two methods. First ifthe command conforms to format 142, the responder station is addressedwhen the result of ARBITRATION NUMBER logically ANDed with MASK isbitwise identical to BRANCH. ARBITRATION NUMBER is the current contentsof a particular register in register array 66. MASK and BRANCH arevalues received in the command and stored in register array 66. Logicaloperations and comparisons are performed by ALU 72 which produces A=Bsignal 82. If A=B signal 82 is not asserted, state 314 is entered.Responder station 40 may remain in state 314 until a predetermined timeelapses. Responder station 10 re-enters idle state 310, after thepredetermined time elapses.

To illustrate the importance of such a delay, suppose that commander andresponder stations employed radio transceivers for network interfaces 60and 26. Then, suppose responder station 40 is within range of twocommander stations 10 and 34, but commander stations 10 and 34 are outof range of each other. When commander stations 10 and 34 validlyproduce back to back commands, the delay interposed by state 314prevents responder station 40 (not addressed by commander station 10 inthe first occurring command) from responding to commander station 34 inthe second occurring command. Without the delay, a collision could occurthat may confuse commander station 10.

A second way to determine whether responder station 40 has beenaddressed applies for commands having formats 194 and 196. Accordingly,responder station 40 is addressed when ARBITRATION NUMBER, retained inregister array 66, is bit-wise identical to ARBITRATION NUMBER asreceived in the command. Comparison is performed by ALU 72 whichproduces A=B signal 82. If A=B signal 82 is not asserted, state 314 isentered as already described. Otherwise, transition is made to decodestate 316.

Decode state 316 follows address check state 312 in response to A=Bsignal 82. If the command opcode is not recognized then no responsestate 314 is entered. For some commands, a further condition such as thestate of locked-bit 88, if not met will cause the command to be treatedas not recognized. Each recognized command opcode causes microcodeexecution to begin at a section of microcode for the purpose ofdirecting microsequencer operations to process the particular receivedcommand. Four commands are illustrated as separate states 318 through324 and other commands are illustrated in summary by pseudo state 326.

When the opcode for command IDCG has been received, state 318 is enteredfor identify, clear, and generate operations. An lOR response (accordingto FIGS. 8 and 9 of the '551 patent) is selected, locked-bit 88 iscleared, the content of random number generate 90 is stored in registerarray 66 as ARBITRATION NUMBER, and transition is made to state 328.

When the opcode for command lDG has been received and locked-bit 88 isnot set, state 320 is entered for identify, and generate operations. AnlOR response is selected and a new ARBITRATION NUMBER is generated asalready described for state 318. Transition is then made to state 328.

When the opcode for command IDC has been received, state 322 is enteredfor identify and clear operations. An lOR response is selected andlocked-bit 88 is cleared. Transition is then made to state 328.

When the opcode for command 10 is received and locked-bit 88 is not set,state 324 is entered for an identify operation. An lOR response isselected. Transition is then made to state 328.

When the opcode for other commands (including RD and WD) is received,locked-bit 88 may be set and other functions may be performed. Otherfunctions may include writing data to memory 64, writing data toregister array 66, altering the state of registers including flagregister 84, and other operations controlling responder stationconfiguration and operation. Transition is then made to state 328.

Upon transition to state 328, the response selected by a prior state isgenerated according to FIGS. 7, 8, and 9 of the '551 patent andbroadcast. In one embodiment, the response is broadcast as it is beinggenerated. Transition to idle state 310 is made, after broadcasting theresponse. Note that responder station 40 does not wait for clear mediumprior to broadcasting the response. According to the present inventionas described in the '551 patent, collision detection by responderstations is not necessary to accomplish uninterrupted communication.

The ARBITRATION NUMBER generated by responder station 40 and the BRANCHand MASK numbers chosen by commander station 10 operate to establishuninterrupted communication. We now turn to a further explanation of themethod used by commander station 10 to choose BRANCH and MASK values.

FIG. 12 of the '551 patent is a binary tree diagram of BRANCH values andMASK values chosen by a commander station. A tree is a type of graphicrepresentation. There are several types of trees known in mathematicsand computer science. The tree depicted is a binary tree where a nodecan have two branches, shown descending left and right from a node. Eachnode corresponds to a unique combination of values for BRANCH and MASK,which are nbit binary numbers having the same precision. As illustrated,BRANCH and MASK are n-bit binary numbers. In a communication system forairport baggage handling, a-bit numbers would be used. The precisionemployed for BRANCH and MASK must be identical to the precision selectedfor ARBITRATION NUMBER generated by responder station 40.

Recall that responder station 40 uses the expression ARBITRATION NUMBERAND MASK=BRANCH to determine if it is addressed, where ARBITRATIONNUMBER is the value retained in register array 66 from random numbergenerator 90. When MASK is 0 and BRANCH is 0 all values of ARBITRATIONNUMBER satisfy the expression, i.e. all responder stations coupled tocommon medium 32 conclude they are addressed. On the other hand, if MASKhas a ‘1’ bit in every position, then the expression is satisfied foronly one value of ARBITRATION NUMBER.

When MASK is arranged with ‘0’ and ‘1’ bits, the expression is satisfiedby a group of values for ARBITRATION NUMBER, and potentially a group ofresponder stations could conclude they are addressed. Note for aresponder station to be addressed, BRANCH at bit position ‘q’ must be‘0’ when MASK at bit position ‘q’ is ‘0’, for all values of ‘q’. WhenMASK at bit position ‘q’ is ‘1’, BRANCH can take two values for that bitposition which correspond to the left and right branches of a binarytree.

At the first level of the tree, nodes 702 and 703, MASK is ‘1’ in bitposition ‘r’. The corresponding bit position of BRANCH is ‘0’ at node702 and ‘1’ at node 703. At the second level of the tree, nodes 704through 707, MASK is ‘1’ at bit positions ‘r’ and ‘s’. For example, thevalue for BRANCH at node 707 is the parent node BRANCH value (001 atnode 703) modified by forcing a ‘1’ (for the right-hand branch) at bitposition ‘s’, hence 011. In like manner, the BRANCH and MASK values forany node in the tree can be determined. In FIG. 12 of the '551 patentMASK bit positions have been given in an order right to left. Any otherorder of bit positions would be equivalent. Methods for choosing firstand subsequent values for BRANCH and MASK can now be explained in termsof traversing from node to node on a binary tree.

When commander station 10 broadcasts a request for identification (anID, IDC, lDG, or IDCG command) one of three events can occur. BRANCH andMASK values given at a particular node that represents a first group ofresponder stations. First, commander station 10 could receive noresponse from which it could conclude that no responder station in thefirst group is currently coupled to the common medium 32. Second, aproper response could be received. From that event, commander station 10could conclude that only one responder station in the first group iscurrently coupled to common medium 32. Third, from an improper responsereceived, commander station 10 could conclude that a collision of morethan one response occurred. An improper response could be caused by, forexample, weak coupling, high noise levels, or weak received signals.However, these causes can be treated in the same way as a collision tosimplify the commander station protocol without substantially degradingsystem performance for applications including airport baggage handling.Therefore, an improper response simply merits further search.

An efficient search for the identity of each of several respondingresponder stations is equivalent to an efficient search for the leavesof a binary tree. A leaf is a node having no further branches. When useof the values for BRANCH and MASK at a node produces no collision, thenode is a leaf. Tree search methods are easily implemented using knowncomputer programming methods.

Tree search methods are essentially of two types, breadth first anddepth first. A particular communication system application may use onemethod or the other to optimize commander station computing time andmemory space objectives. An explanation of these methods using theprogramming language PASCAL is given by E. Horowitz and S. Sahni in“Fundamentals of Data Structures in PASCAL” pp 203-265 and 326-332published by Computer Science Press Inc., Rockville, Md. (1984),incorporated herein by reference.

Suppose that two responder stations 40 and 36 and one commander station10 are coupled to common medium 32. The binary tree in FIG. 12 of the'551 patent illustrates a sequence of BRANCH and MASK values used bycommander station 10 to identify responder stations. Timing diagrams inFIGS. 13 and 14 of the '551 patent illustrate the same sequence showingdecisions at commander station 10 decision blocks (according to thecommander station method of FIG. 10 of the '551 patent) and responderstation control signals (according to the responder station method ofFIG. 11 of the '551 patent) as commander station 10 establishesuninterrupted communication with each responder station.

Beginning at FIG. 10 of the '551 patent block 210, FIG. 12 of the '551patent node lOt and FIG. 13 of the '551 patent time 810, commanderstation 10 chooses BRANCH=OOO and MASK=OOO, calling for all responderstations to respond. At time 815, responder station 40 has determinedthat it is addressed, has cleared its locked-bit 88, has generatedARBITRATION NUMBER 101, and has begun transmitting response lDR.Simultaneously, responder station 36 has determined that it has beenaddressed, has generated ARBITRATION NUMBER 111, and has beguntransmitting response lDR. Also, at time 815, simultaneous transmissionscollide on common medium 32.

At time 820, commander station 10 at block 226 chooses node 702 havingBRANCH=OOO and MASK=001. Responder station 40 is not addressed becauseARBITRATION NUMBER (101) ANDed with MASK (001) yields 001 which is notequal to BRANCH (000). Similarly, responder station 36 is not addressed.Neither station responds. At time 826, time out at block 218 occurs.

At time 830 and block 226, a third group of responder station addressesis chosen. From FIG. 12 of the '551 patent the appropriate group isspecified by traversing the tree according to a search method. If abreadth first search were used, all nodes at the same level would bevisited before testing at a deeper level. Hence, node 703 would be next.If a depth first search were used, search would proceed upward from node702 (because it is a leaf) and then downward from the first node havingan untested branch. Hence, up to node 701 and down to node 703. As arefinement to either method, node 703 can be skipped because a collisionat node 701 and no response at node 702 implies a collision at node 703without testing. A depth first search would now traverse from node 703directly to node 706. A breadth first search would first consider nodes704 and 705 and conclude not to visit them because each is a descendentfrom a leaf node.

Having selected node 706 at time 830, commander station 10 broadcasts an10 command with BRANCH=001 and MASK=011 at block 216. At time 835responder, station 40 has determined that it is addressed and has beguntransmitting response IDR. Simultaneously, responder station 36determines it is not addressed and remains in state 314. At time 840,shown on FIGS. 13 and 14 of the '551 patent, commander station 10 hasreceived the response from responder station 40 as a proper message,concluded that only one responder station responded, derived receivedARBITRATION NUMBER (101), set BRANCH to received ARBITRATION NUMBER, setMASK to all 1's so that a responder station must match ARBITRATIONNUMBER (101) in all bit positions to respond, and begins to performblocks 232 through 244 in FIG. 10 of the '551 patent. At time 845,responder station 40 has determined that it is addressed, has decoded aread command, has set its locked-bit 88 in state 326, and has begungenerating the read response in state 328. At time 850, commanderstation 10 has received the response as a proper message. Thus,commander station 10 has conducted a first two-party uninterruptedcommand-response scenario from time 840 to time 850 with one responderstation.

The search by commander station 10 for another responder stationproceeds from block 244 to block 224 in FIG. 10 of the '551 patent. Atblock 226, another node from FIG. 12 of the '551 patent is selected.Having elicited a proper response at node 706, the depth first searchproceeds up to the first node having an untested branch, here node 703.Then, down the untested branch to node 707. Having selected node 707 attime 850, commander station 10 broadcasts an 10 command with BRANCH=011and MASK=011 at block 216. At time 855, responder station 36 hasdetermined that it has been addressed and has begun generating an lORresponse. At time 860, the response is received by commander station 10as a proper message. After time 860, events proceed in a manner similarto events from time 840 to time 850, as commander station 10 conducts asecond two-party uninterrupted command-response scenario with a secondresponder station.

At block 224, following the uninterrupted scenario, commander station 10can conclude that all groups have been tested. On a depth first search,a proper response or no response at a node having BRANCH=MASK indicatesall groups have been tested. On a breadth first search, all groups havebeen tested when an investigation of all levels up to the level havingall MASK bits set to ‘1’ yields no node that is not descendent from aleaf node.

In a branch/mask embodiment of the type described above, a responderstation concludes that it has been addressed when ARBITRATION NUMBERlogically ANDed with MASK is equal to BRANCH. Two other types ofembodiments will now be described that lie within the scope and spiritof the present invention as described in the '551 patent. First, in anexample of a high/low embodiment, BRANCH and MASK (as shown in format142) are replaced with HIGH LIMIT and LOW LIMIT. Each of these limitvalues has the same precision as the MASK value. Using these limitvalues, responder station 40 concludes that it is addressed when HIGHLIMIT is greater than or equal to ARBITRATION NUMBER, and LOW LIMIT isless than or equal to ARBITRATION NUMBER. Second, in an example of alimit/bound embodiment, BRANCH and MASK (as shown in format 142) arereplaced with a single LIMIT value having the same precision as MASK.Using a value called BOUND which by design choice may be 0 or themaximum permitted by the precision of LIMIT, responder station 40concludes that it is addressed when ARBITRATION NUMBER is between BOUNDand LIMIT, inclusive of both BOUND and LIMIT values.

An example of a limit/bound embodiment is implemented with a structuresimilar to that already described for the branch/mask embodiment.Subtraction capability or equivalent must be added to ALU 72. Operationof microsequencer 42 must be revised to perform the arithmeticoperations outlined above in state 312 shown on FIG. 11 of the '551patent. The high/low embodiment is implemented with the structurealready described for the limit/bound embodiment.

In FIG. 10 of the '551 patent (blocks 210 and 226) commander station 10specifies a group of responder station addresses. For a branch/maskembodiment, a method using the binary tree of FIG. 12 of the '551 patenthas already been discussed. For a high/low embodiment, a similar binarytree (not shown) with HIGH and LOW values at each node is used. At theroot node, LOW is 0 and HIGH is the maximum value permitted by theprecision of the value HIGH. At a node on the lower left from a parentnode, the value of LOW is the value of LOW at the parent node and thevalue of HIGH is a value ½ the value of HIGH at the parent nodediscarding a remainder, if any. At a node on the lower right from aparent node, the value of HIGH is the value of HIGH at the parent nodeand the value of LOW is ½ the value of HIGH at the parent node plus one.Although a binary tree has been described, a tree having more than twobranches at each node can be employed to practice the present inventionas described in the '551 patent as is readily apparent to those skilledin the art. Trees with varying number of branches at each node can alsobe employed. Operation of the high/low embodiment is otherwise identicalto operation of the branch/mask embodiment already discussed.

In a limit/bound embodiment, the method used to specify a group ofresponder station addresses is similar to the method described for ahigh/low embodiment with a minor variation in the tree. When BOUND iszero, then the value for LOW is not used and the value for HIGH is usedas the LIMIT value at each node. When BOUND is a maximum value, then thevalue for HIGH is not used and the value of LIMIT at each node is thevalue of LOW. Operation of a limit/bound embodiment is otherwiseidentical to operation of a branch/mask embodiment already discussed.Note that the command at block 232 on FIG. 10 of the '551 patent setslocked-bit 88 to prevent unnecessary collisions when an 10 command usingLIMIT is broadcast subsequently at block 228.

FIG. 15 of the '551 patent is a fibonacci tree diagram for use in anexample of an embodiment of the type already described as limit/bound.An advantage of using a fibonacci tree is that the LIMIT value for anode descendent from a parent node can be derived without amultiplication or division operation. In systems where it is desirableto improve calculation speed or reduce the complexity of circuitry andsoftware at commander station 10, the fibonacci tree is used. Animplementation of a high/low embodiment using a fibonacci tree similarto FIG. 15 of the '551 patent is within the ordinary skill of thesystems design and programming arts.

As described in several embodiments above, a commander station canquickly determine the identity of all responder stations coupled to acommon medium at a given time. After the identity of a responder stationhas been determined, a commander station can conduct uninterruptedcommunication at any subsequent time using the responder station'sARBITRATION NUMBER. Since the ARBITRATION NUMBER is not unique, there issome risk that at a subsequent time, more than one responder stationhaving a given value for ARBITRATION NUMBER may become coupled to thecommon medium. For increased accuracy, use of a unique responder stationidentity, such as the TAG field in format 146 of FIG. 6 of the '551patent, may be used for subsequent communication.

When more than one commander station is coupled to a common medium, itis possible for one commander station to thwart the objective of asecond commander station. For example, when commander station 10 isattempting to identify all responder stations and commander station 34issues an 10CG command, commander station 10 may subsequentlyincorrectly conclude that all responder stations have been identified.Several methods of preventing this incorrect conclusion are available tothose skilled in communication and computer programming arts. Exemplarymethods include enabling a commander station to monitor commands issuedby another commander station to avoid inappropriate conclusions;enabling a commander station 10 to record the TAG fields sent inmessages to another commander station and communicate directly with eachsuch responder station, perhaps prior to and so simplifying, the task ofidentifying all responder stations; modifying the communication protocolused among commander stations; and causing a second commander station todelay its own attempt to identify all responder stations until after atime sufficient for a first commander station to identify all responderstations. The latter suggestion is practical using the media accesscontrol scheme of the present invention as described in the '551 patent.It is practical because the time required to identify a worst-casepopulation of responder stations can be predetermined.

The present invention as described in the '551 patent can be implementedin several alternate embodiments. As already discussed, variousalternatives are available for common medium 32 including all media thatsupport forms of electromagnetic energy, all sound, vibration, andpressure wave conducting media, and all media capable of transportingvariation in chemical concentration, to name a few. If a medium otherthan radio communication is selected as an embodiment of the presentinvention as described in the '551 patent, variations in networkinterface 26 and 60 can be made by those skilled in the arts that applyto the selected medium. Appropriate signal sensors and generators arewell known in applications including measurement and control apparatus.Packet synchronization techniques, packet formats, error detectiontechniques, and error correction techniques may vary or be omitted as amatter of design choice depending on the need for receiversynchronization, the signal to noise properties of the selected media,and the desired simplicity of network interfaces.

Another group of alternative embodiments uses various means to specify aset of responder station identities or designations. The embodimentsdescribed above employ an ARBITRATION NUMBER selected from apredetermined range of numbers and expressed as part of a message. Forexample, alternate sets of designations include a set of operatingmodes, a set of modulation techniques, and a range of values used toshift in time all or a portion of a command. Various alternatives arealso available for specifying (i.e. addressing) a subset ofdesignations. The branch/mask, high/low, and limit/bound subsetaddressing techniques can each be applied to one or more parametricquantities related to the above mentioned set designations. For example,if onemember of the set is characterized by a bandwidth, a channelfrequencies, a phase variation, or a duration in time, then a range ofeach of these parameters could be described by a branch/mask pair ofvalues.

Various alternatives for transmitting the command signal are within thescope of the present invention as described in the '551 patent. In theembodiments described at length above, the BRANCH and MASK values in themessage format characterize the transmitted command signal according toa subset of responder stations to which the command is directed. Inaddition to the variations in modulation already described, thetransmitted signal can be characterized by variation in the spreadspectrum chip sequence or initial code within a chip sequence whenspread spectrum transmission is employed.

Other characteristics of a command signal can be used to limit or expandthe subset of responder stations to which the command is directed. Forexample, the operation of commands including RO and WR to set locked-bit88 and the operation of commands including IDG and IDCG to conditionallyclear locked-bit 88 show how the command opcode can be used tocharacterize a command signal according to a selected subset or addressrange. Alternatively, modulation variations, timing variations, or othermessage content variations could also be used to set or clear anequivalent of the locked-bit function.

Various means are suitable for use by a responder station to determinewhether it is addressed by, i.e. whether it is a member of the subsetindicated by, a command signal. Several arithmetic comparison techniquesbased on message content have been described above. Other means, basedon whether the signal received by the responder is received withouterror, are appropriate when variations in modulation are used to addressa subset of responder stations. For example, received signal strengthbelow a threshold over one or more frequencies or at a particular timecould cause commands to be received or rejected. Similarly, operation offunctions similar to locked-bit 88 as already described and variation inspread spectrum chip sequence could be used to cause commands to bereceived or rejected.

Within the scope of the present invention as described in the '551patent, each responder station includes means for establishing a selfdesignation. In the embodiments discussed at length above, the selfdesignation is determined by a random number generator, held in aregister, and included in a response packet. Alternative techniquesinclude various means for sampling a random process to acquire an analogparametric value and using either a digital or an analog value tocontrol the functions of network interface 60.

Network interface 60 can be constructed and operated in severalalternative embodiments to transmit a response packet in a waycharacterized by the responder station self designation. All of thefollowing variations could be used in embodiments that fall within thescope of the present invention as described in the '551 patent:variations in the modulation technique, including variation within arange of values used to shift in time all or a portion of a response;variation in the spread spectrum chip sequence or initial code within achip sequence when spread spectrum transmission is employed; variationin message content including preamble, postamble, response typeindicator e.g. IDR, RDR, and WDR, register contents, status andlocked-bit information; and variation based on signal rejectionincluding variation in bandwidth, channel frequencies, signal phasevariations, signal duration, or variation in the redundancies used todetect or correct transmission error.

Another group of alternative embodiments uses alternative means forselecting a subgroup in response to collision detection. The tree searchmethod that was described as part of the commander station protocol canbe implemented in various ways depending on the selected representationof the tree in commander station memory 18. Binary trees have beendescribed above. Other tree structures including n-ary trees could beemployed to perform the commander station identification function in anequivalent manner. Depending on the type of tree selected forrepresentation, the use of strings, arrays, stacks, pointers, linkedlists, or derivative memory organizations are feasible and equivalent.Finally, tree search methods include depth first, breadth first, andcombinations of both depth and breadth searching.

Each computer used as part of commander station 10 and as part ofresponder station 40 includes hardware and software designed to conductthe protocol shown and described in S. 10 and 11 respectively of the'551 patent. Variations in the extent and complexity of hardware andsoftware are well known by designers of ordinary skill in communicationand computer arts. Equivalent hardware can include the general purposecomputer such as an IBM PC; a calculator, such as an HP21C; the specialpurpose computer, such as application specific automated controllersused in weighing systems; the microprocessor based system, such as acircuit using an Intel 8048; the microsequencer based system usingprogrammable devices and logic devices; and the integrated circuit orchip set, such as developed from a cell library using semiconductordevice design techniques. Variations in the extent and complexity ofsoftware compatible with one or more of the above mentioned hardwarevariations are also well known by the programmer of ordinary skill.

The systems designer of ordinary skill chooses to implement each controlfunction in either hardware or software or a combination of both. Acomputer is said to conclude a certain result when it has determined thestate of a control function. When a control function is implementedusing a computer system, variations in the form of the result of thecontrol function are well known. For example, a parameter that resultsfrom a first control function and is relied upon by a second controlfunction can take the form of a signal when the second control functionis in part hardware or the form of a pointer, value, or symbol stored ina register or memory when the second control function is in partsoftware.

The present invention as described in the '551 patent has been describedin the preferred embodiment. Several variations and modifications havealso been described and suggested. Other embodiments, variations, andmodifications known to those skilled in the art may be implementedwithout departing from the scope and spirit of the invention as recitedin the claims below.

1. A method of testing the RF communication operation of an RFtransponder, comprising the steps of: providing a sheet characterized byfirst and second opposite faces and a thickness; mounting on the sheetan RF transponder that includes a transponder RF antenna; positioning afirst RF shield so as to abut the first face of the sheet; positioning asecond RF shield so as to abut the second face of the sheet, the secondRF shield being in the shape of a cup having a mouth abutting saidsecond face, wherein the first and second RF shields are positioned sothat the first and second RF shields together form a closed cavity whichcompletely surrounds and encloses the transponder RF antenna exceptwhere the thickness of the sheet separates the first RF shield from themouth of the second RF shield, wherein said thickness is sufficientlysmall so that the first and second RF shields prevent any RF signalswithin the cavity from radiating outside the cavity; positioning a testfixture RF antenna within the cavity; transmitting an RF signal from thetest fixture antenna; detecting a response by the transponder to the RFsignal; and subsequently removing the transponder from proximity to thefirst and second shields and the test fixture RF antenna, so that noshielding obstructs the transponder RF antenna from sending andreceiving RF radiation at any angle.
 2. A method according to claim 1,wherein the cavity encloses the entire RF transponder.
 3. A methodaccording to claim 1, wherein the sheet has no shielding mounted thereonthat obstructs RF radiation from the transponder RF antenna.
 4. A methodaccording to claim 1, wherein: the first RF shield is in the shape of acup having a mouth abutting the first face; and the step of positioningthe second RF shield further comprises aligning the mouth of the secondshield with the mouth of the first shield.
 5. A method according toclaim 1, wherein the step of positioning the test fixture RF antennawithin the cavity comprises: mounting the test fixture RF antenna to asurface of one of the two RF shields; connecting an RF transmission lineto the test fixture RF antenna; and passing the transmission linethrough an opening in said one RF shield to extend outside the cavity.6. A method according to claim 1, further comprising the step of:fabricating the sheet to include electrically conductive materialadjacent the mouth of the second RF shield so as to improve RF shieldingof the cavity.
 7. A method according to claim 1, wherein the RF signalis transmitted at a predetermined wavelength, and wherein the RF shieldsare dimensioned to improve the gain of the cavity at that wavelength. 8.A method according to claim 1, wherein the RF signal is transmitted at apredetermined wavelength, and wherein the RF shields are dimensioned sothat the cavity resonates at that wavelength.
 9. A method of testing theRF communication operation of a plurality of RF transponders, comprisingthe steps of: providing a sheet characterized by first and secondopposite faces and a thickness; mounting on the sheet a plurality of RFtransponders, wherein each transponder includes a transponder RFantenna; positioning a first test fixture section having a first RFshield so that the first RF shield abuts the first face of the sheet;positioning a second test fixture section so as to abut the second faceof the sheet, wherein: the second test fixture section includes aplurality of RF shields, each RF shield in the second test fixturesection is in the shape of a cup having a mouth abutting said secondface of the sheet, the first and second test fixture sections so thateach RF shield in the second test fixture section encircles acorresponding one of the transponder RF antennas so as to form, incombination with the first RF shield, a closed cavity that completelysurrounds and encloses said corresponding transponder RF antenna exceptwhere the thickness of the sheet separates the first RF shield from themouth of said RF shield in the second test fixture section, and saidthickness is sufficiently small so that the first and second RF shieldsprevent any RF signals within the cavity from radiating outside thecavity; positioning within each cavity a corresponding test fixture RFantenna; transmitting an RF signal from each test fixture antenna;detecting a response by each transponder to the RF signal transmitted byits corresponding test fixture antenna; and subsequently removing eachtransponder from proximity to the first and second test fixture sectionsand the test fixture RF antennas, so that no shielding obstructs eachtransponder RF antenna from sending and receiving RF radiation at anyangle.
 10. A method according to claim 9, wherein the each cavityencloses the entire corresponding RF transponder.
 11. A method accordingto claim 9, wherein: the first RF shield is in the shape of a pluralityof cups so that each cup has a mouth abutting the first face of thesheet; and the step of positioning the second RF shield furthercomprises aligning each mouth of the second shield with a correspondingmouth of the first shield.
 12. A test fixture for testing the RFcommunication operation of an RF transponder which is mounted on a sheetwhich extends beyond the perimeter of the transponder, the RFtransponder having an antenna for receiving RF signals, comprising:first and second RF shields, the second RF shield being in the shape ofa cup having a mouth; an alignment mechanism for positioning the firstand second RF shields to abut opposite sides of the sheet so that themouth encircles the transponder antenna and so that the combination ofthe first and second RF shields forms a closed cavity completelysurrounding and enclosing the transponder antenna except where the sheetseparates the two RF shields, wherein the distance by which the sheetseparates the two RF shields is small enough to prevent any RF signalswithin the cavity from radiating outside the cavity; and a test fixtureRF antenna mounted within the cavity.
 13. A test fixture according toclaim 12, further comprising: a test fixture RF transmitter having anoutput connected to the test fixture RF antenna so that the RF antennaradiates RF signals to the transponder RF antenna; and a test fixture RFreceiver having an input connected to the test fixture RF antenna sothat the RF receiver receives any responses transmitted by the RFtransponder in response to said RF signals.
 14. A test fixture accordingto claim 12, wherein the cavity encloses the entire transponder.
 15. Atest fixture according to claim 12, wherein: the first RF shield is inthe shape of a cup having a mouth abutting the first face; and thealignment mechanism aligns the mouth of the second shield with the mouthof the first shield.
 16. A method according to claim 12, furthercomprising: an RF transmission line connected to the test fixture RFantenna; wherein the transmission line extends through an opening in oneof the RF shields so as to extend outside the cavity.
 17. A test fixtureaccording to claim 12, further comprising a test fixture RF transmitterfor providing to the transponder antenna RF test signals having apredetermined wavelength, wherein the first and second RF shields aredimensioned to improve the gain of the cavity at that wavelength.
 18. Atest fixture according to claim 12, further comprising a test fixture RFtransmitter for providing to the transponder antenna RF test signalshaving a predetermined wavelength, wherein the first and second RFshields are dimensioned so that the cavity resonates at that wavelength.19. A test fixture for testing the RF communication operation of aplurality of RF transponders mounted on a sheet, each RF transponderhaving an RF antenna, comprising: a first test fixture section includinga first RF shield; a second test fixture section including a pluralityof RF shields each of which is in the shape of a cup having a mouth; analignment mechanism for positioning the first and second test fixturesections to abut opposite sides of the sheet so that each RF shield inthe second test fixture section encircles a corresponding one of thetransponder antennas so as to form, in combination with the first RFshield, a closed cavity that completely surrounds and encloses saidcorresponding transponder RF antenna except where the sheet separatesthe first RF shield from the mouth of said RF shield in the second testfixture section, wherein the distance by which the sheet separates thefirst RF shield from each RF shield of the second test fixture sectionis small enough to prevent any RF signals within each cavity fromradiating outside that cavity; and a test fixture RF antenna mountedwithin each cavity.
 20. A test fixture according to claim 19, wherein:the first RF shield is in the shape of a plurality of cups so that eachcup has a mouth abutting the sheet; and the alignment mechanism alignseach mouth of the second shield with a corresponding mouth of the firstshield.
 21. A system for performing radio frequency communications, thesystem comprising: one or more first antennas; an interrogatorcommunicatively coupled to the one or more antennas to transmit a firstcommand; and a radio frequency identification (RFID) tag comprising: oneor more second antennas, at least one of the one or more second antennasbeing a dipole antenna; a battery; a random number generator to generateone or more random numbers; and processing circuitry electricallycoupled to the one or more antennas, the battery, and the random numbergenerator, the processing circuitry configured to operate in a firststate and a second state that uses more battery power than the firststate, and a second state that uses more battery power than the firststate, the processing circuitry further configured to transition fromthe first state to the second state upon receipt of the first commandand to communicate a message to the interrogator at a time based atleast in part on a first random number generated by the random numbergenerator.
 22. The system of claim 21, wherein the message comprises atleast one random number generated by the RFID tag.
 23. The system ofclaim 22, wherein the interrogator is further configured to transmit atleast an additional command including the at least one random number toidentify the RFID tag.
 24. The system of claim 21, wherein theinterrogator is further configured to transmit at least a second commandthat includes a selection indicator, and the processing circuitry isfurther configured to communicate the message if the selection indicatorcorresponds to one or more bits stored on the RFID tag.